The present invention relates to disk shaped recording medium and a disk drive unit used suitably for such a disk shaped recording medium.
This invention technically relates to the U.S. patent application Ser. No. 144,970, based upon a Japanese priority application which was not yet published as of the filing date of the present application. The above U.S. application is owned by the assignee of the present invention.
FIG. 15 shows the basic arrangement of a magnetic disk 1. As shown in the figure, the magnetic disk 1 has each track partitioned into a plurality of sectors, and each sector is made up of a plurality of segments Conventionally, a compact magnetic disk unit that drives such a magnetic disk as the magnetic disk 1 employs the scheme of embedding the servo information at the positions where the disk surface is equally divided on the magnetic disk 1 (embedding servo system). This system falls into two types, i.e., the sector servo system and sample servo system. In the sector servo system, several tens of servo information areas (which will be termed simply "servo areas" hereinafter) are formed at the positions where the disk circuit is equally divided and the servo information is recorded there, whereas in the sample servo system, several hundreds of servo areas are formed as a convex/concave pattern.
Generally, in the sector servo system, a clock signal for providing the data recording/reproduction timing is extracted from the reference signal (preamble) recorded in,the data area at the time of data writing or from a data string itself. Namely, the sector servo system can be said to be of the "self-synchronization type" In contrast, in the sample servo system, a clock signal provided for the data recording/reproduction timing is retrieved from a clock pattern that has been formed in advance with a physical means or magnetic means. Namely, the sample servo system can be said to be of the "external synchronization type".
FIG. 16 shows the format of sector adopted by disk units of the sample servo type. Each segment is basically partitioned into a data recording area for recording data and a servo area for recording the servo control signal as shown in the figure. In addition, the leading segment of each sector has the formation of an ID area for recording the ID (attribute information of the sector) of that sector. The capacity of the data area (capacity of user data) per sector is generally 512 bytes in the case of a compact magnetic disk, and an error correction code (ECC) is appended to the area.
A gap area, where nothing is recorded, is formed between the ID area and the data area. The ID is read out of the ID area and, after the consistency of it with the ID of the sector has been checked, data is recorded or reproduced in the succeeding data area. In this case, it takes some time to confirm the consistency of the ID read out of the ID area with the ID of the target sector. This gap area is provided to allow time for making the consistency determination. A "pad" is null data used during recording for recording correctly all codes left in the modulation circuit at the end of ECC, and is also used at reproduction for stably reading ECC up to the end.
FIGS. 17(a) and 17(b) show in detail the format of the servo area of a disk used by disk units of the sample servo type. The servo area has the formation of a clock pattern, access pattern and fine pattern as shown in the figure. FIG. 17(b) is a cross-sectional diagram of the track center taken along the dashed line b of FIG. 17(a). Reproduction of these patterns with the magnetic head reproduces isolated waveforms at the leading edge and trailing edge of the patterns. These patterns are formed by, for example, etching off part of a magnetic layer 1b which is formed on a substrate 1a. These patterns are d.c. magnetized in the lateral direction with a magnetic head.
Clock patterns are formed consecutively on lines of radius in a virtually radial arrangement on the magnetic disk 1, and a clock necessary for recording and reproduction is generated based on the clock pattern as a reference position. The time point of the peak of the reproduced isolated waveform of the clock pattern provides the data, system and servo system with clock information which is synchronous with the disk rotation.
The magnetic head positioning servo has two modes of track seek and tracking. The former mode is to move the head to a target track, and the latter mode is to position the head accurately at the center of the target track. The access pattern is used at track seeking. This pattern having a unique length and disposition for each track is created from the track address by Gray-like coding or the like. The pattern is formed by d.c. magnetizing the magnetic layer as mentioned above.
The fine pattern is used for tracking control. The fine pattern includes four pattern types of X, Y, A and B. Pattern X is formed on a track n and every second track from it (tracks n.+-.2, n.+-.4, . . . ). Pattern Y is formed on the tracks adjacent to the track n and every second track from them (tracks nil, n.+-.3, . . . ). Pattern A is the same as the pattern X, but is formed by being shifted inwardly by a half track pitch. Pattern B is the same as the pattern Y, but is formed by being shifted inwardly by a half track pitch.
Formed between the data area and the clock pattern on the upstream side of the servo area (left side in FIG. 17(a)) is a gap where no control signal is recorded. After data has been recorded in the data area, when the servo area comes, it is necessary to reproduce the servo control signal recorded in this section. On this account, it is necessary to switch from the recording system to the reproduction system at the timing of the transition from the data area to the servo area.
However, the amplifier of the reproduction system cannot settle to the steady state immediately following the switching, and it takes some time for the normal operation. For dealing with this matter, the gap is provided for allowing time until the reproduction amplifier settles. In the case of the sample servo system, in order to generate a high-accuracy clock and retrieve sufficient servo signals, there are disposed servo areas at about several hundreds to several thousands of positions per track at a constant interval. Accordingly, many servo areas are cut in to a sector.
FIG. 18 shows the format of sector adopted by disk units of the sector servo type. Also in this case, as in the sample servo type basically, a sector is formed of 512-bytes of user data to which is added a sector ID and ECC data. However, the sector servo system needs various synchronizing marks because it is of the self-synchronization type. It further necessitates a long gap for absorbing the difference in length due to the positional vibration and rotational speed error. Specifically, an ID preamble, ID sync mark (ID sync), pad, data preamble, data sync mark (data sync), pad, and inter-sector gap are needed.
The ID preamble and data preamble, which are for the recovery of PLL and establishment of bit synchronization, need 10 to 20 bytes each. The ID sync and data sync marks, which are for the establishment of byte synchronization, are formed of special patterns of several bytes. These marks are required before the ID (ID sync) and before logical data (data sync). The pad, which is null data used at recording for recording correctly all codes left in the modulation circuit at the end of ECC and used at reproduction for reading ECC up to the end stably, is required in the rear of the ID and in the rear of the logical data.
The sector servo system is inherently designed to place a servo area between logical sectors, and the number of logical sectors and the number of servo areas are conventionally equal (e.g., 30 to 80). In this case, the number of sectors is constant for any track throughout track positions on the disk, resulting in a significantly decreased memory capacity with respect to the physical capacity of the outer tracks due to low line density recording.
However, in order to meet the demand of increased storage capacity in recent years, the zone bit recording (ZBR) system, in which the disk surface is partitioned radially into several zones each having a specific constant line density (CLV), is becoming prevalent with the intention of reducing the difference of line density throughout track positions. In the sector servo system, with the ZBR system being adopted, the logical sector and servo area have no relation with each other, and a servo area cuts in between logical sectors. In this case, the synchronization of data between servo areas is lost, and it becomes necessary to place a preamble (data preamble) and recurrent sync (re-sync) mark (data sync) after a servo area has passed.
FIG. 19 shows in detail the format of the servo area of a disk used by disk units of the sector servo type. Servo areas are arranged radially at a constant interval, as in the case of the sample servo system. The number of servo areas is 30 to 80, which is about 1/10 of the sample servo system, and a servo area has a length that is twice or more of the sample servo system. The servo area includes an AGC burst, servo header, clock sync, pattern sync, index, access pattern and fine pattern. The AGC burst is an area provided for fixing the gain across the servo pattern. The fine pattern is not located at the track center and automatic gain control does not work normally for the fine signal, and therefore the gain needs to be fixed.
The servo header is a pattern indicative of a servo area, and it is a pattern that does not appear in the data area. The clock sync is the sync signal for the servo clock. The pattern sync is provided as the reference of time axis for the servo signal detection. The index is to obtain the rotational synchronism and rotational center, and only one index per circuit is provided, for example. The access pattern is to identify a track, and it is a Gray-like code. The fine pattern is a group of burst patterns such as X, Y, A and B. The use of the access pattern and the fine pattern is identical to the sample servo system.
The magnetic disk 1 desirably has as much storage capacity as possible, and for an increased storage capacity, it is necessary at first to increase the surface recording density. At the same time, it is necessary to enhance the efficiency of the data format. For the enhancement of the data format efficiency, the following problems arise in the above-mentioned disks of the sample servo type and sector servo type. Problems of magnetic disk of sample servo type:
A magnetic disk of the sector servo type uses about 5 to 10% of each track for the servo area, whereas a magnetic disk of the sample servo type uses about 10 to 20% of each track for the servo area for the need of an increased number of servo areas. In addition, each servo area needs to be preceded by a gap equivalent to the recording/reproduction mode switching time of the head amplifiers, and therefore it is difficult for a magnetic disk of the sample servo type to enhance the format efficiency. Problems of magnetic disk of sector servo type:
A magnetic disk of the sector servo type uses both the servo clock and data clock created asynchronously with the disk rotation from a preamble (e.g., string of 1's and 0's) written on the disk. For the byte-wise synchronization, it needs a byte synchronizing mark. Accordingly, it needs tens of bytes of area only for the sync information area. In addition, since the data clock is generated asynchronously with the disk rotation, the length of data writing varies due to the unevenness of disk rotation. On this account, longer gaps are needed between a servo area and data recording area and between sectors.
Today's magnetic disk units use zone bit recording in order to increase the storage capacity as mentioned previously. Therefore, servo areas are cut into logical sectors in many zones. At each event, a preamble, synchronizing mark and gap are required. These matters must be considered for each logical sector, and therefore it is inevitable for each logical sector to have an overhead of about 60 bytes or more.
In view of the foregoing situation, the present invention is intended to enhance the efficiency of the data format for a disk of the sample servo type.